The present invention generally relates to the field of integrated circuits, and particularly to a bus driver having temperature compensated circuitry that varies the resistance presented to the bus line.
The Universal Serial Bus (USB) is an industry standard for a cable bus that supports data exchange between a host computer and one or more peripherals. It provides for connection of a personal computer to a data communications line and may be used for port expansion. The major goal of USB was to define an external expansion bus, easy to use and low in cost, which makes adding peripherals to a personal computer as easy as hooking up a telephone to a wall-jack. The external expansion architecture highlights PC host controller hardware and software, robust connectors and cable assemblies, peripheral friendly master-slave protocols, and expandibility through the use of multi-port hubs.
The USB standard, version 2.0, specifies a differential output driver to drive the data signal onto the USB cable at a data rate of 480 Mbps, 40 times faster than USB 1.1 devices. Certain connections for USB 2.0 remain at USB 1.1 data rates, e.g., 12 Mb/sec automatically downshifting to 1.5 Mb/sec for low-speed peripherals. To permit communication with USB 1.1 peripherals, USB 2.0 hubs contain a mechanism that supports data rate matching with the attached peripheral. Any downstream port of a USB 2.0 hub can support attachment of any speed USB device. The USB specifications present challenges to the circuit implementation, especially in CMOS area. The output voltage swings between the two single-ended outputs of a USB driver should be complementary. Furthermore, the output swings between differential high and low should be well balanced, i.e., xc2x110% matching on a signal transition and have a controlled slew rate in order to minimize signal skew, the radiated noise, cross talk and common mode reflection.
The USB version 2.0 is a backwards-compatible extension of the USB 1.1 specification. It uses the same cable, connectors, and software interfaces as USB 1.1, but it also permits the addition of higher performance peripherals, such as video-conferencing cameras may be connected to the bus because of its higher bandwidth, higher speed and higher resolution printers and scanners may be connected to the bus. High density storage devices such as R/W DVD and high capacity CD ROM jukeboxes may also be connected to the USB 2.0 bus. In the worst case, minor modifications may be required in the peripheral interface.
Some specifications, including USB 2.0, require tighter tolerance on termination resistors that a given process is able to attain initially due to process variation, or to maintain due to the effects of voltage and/or temperature changes.
Previously, devices interrupted the operation of the device, either during initialization and/or normal operation, to measure the termination resistance and make corrections or preset the terminating resistance using an external precision resistor. It is problematic to have the operation of the device interrupted in such a manner or require the use of external components.
Therefore, it would be desirable to provide an electronically controlled resistance termination changer which operates over an extended temperature range, which continues to operate during the course of termination resistance adjustment, and which has good granularity.
Accordingly, the present invention is directed to improve the matching characteristics of a bus driver line.
In a first aspect of the present invention, a electronic circuit arrangement for maintaining a given voltage for a given current over an extended temperature range has a first component having a non-zero thermal coefficient of a first polarity and a first magnitude, and a second component having a non-zero thermal coefficient of a second polarity and being of a second magnitude essentially equal to the first magnitude, the second polarity being opposite in sign to the first polarity.
In a second aspect of the present invention, a bus driver circuit has a plurality of sets of switchable subcircuits, each of the sets of switchable subcircuits having an output, the outputs of all the switchable subcircuits being combined to form a single driver bus line, wherein each of the switchable subcircuits contains two subcomponents which have thermal coefficients which are approximately equal in magnitude but opposite in sign.
In a third aspect of the invention, a bus driver circuit has a plurality of sets of switchable subcircuits, each of the sets of switchable subcircuits having an output, the outputs of all the switchable subcircuits being combined to form a single driver bus line, in which each of the plurality of sets of switchable subcircuits contains a differing number of switchable subcircuits.
The present invention describes a method of maintaining one or more terminations within a smaller tolerance window than PVT (process, voltage, temperature) would normally permit. The present invention pairs a resistor and a transistor to work in a stable temperature operation over an extended temperature range. This is achieved through the use of the resistor and the transistor each having a non-negligible temperature coefficient of resistance, equal and opposite in sign to one another, reducing the total variability.
The present invention offers two levels of improved efficiency. In the first level, component parts may be made smaller because their tolerances need not be made so precise. In the second level, duplicating the circuitry with matching characteristics allows one circuit to be operational while the other circuit is tested or dormnant. Switching between the two circuits is performed seamlessly with no interruption of device operation.
The present invention uses fewer adjustment bits and minimizes the temperature control range. A {fraction (1/10)} granularity is provided.
In the present invention, multiple terminations may be trimmed by the same control circuitry, to different tolerances.
It is to be understood that both the forgoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention.